Module 5. Induction motors

Lesson 19

SINGLE PHASE INDUCTION MOTOR

19.1  Single Phase Induction Motor

There are probably more single-phase AC induction motors in use today than the total of all the other types put together. It is logical that the least expensive, lowest maintenance type motor should be used most often. The single-phase AC induction motor best fits this description. As the name suggests, this type of motor has only one stator winding (main winding) and operates with a single-phase power supply. In all single-phase induction motors, the rotor is the squirrel cage type.

The single-phase induction motor is not self-starting (Fig. 19.1). When the motor is connected to a single-phase power supply, the main winding carries an alternating current. This current produces a pulsating magnetic field. Due to induction, the rotor is energized. As the main magnetic field is pulsating, the torque necessary for the motor rotation is not generated. This will cause the rotor to vibrate, but not to rotate. Hence, the single phase induction motor is required to have a starting mechanism that can provide the starting kick for the motor to rotate.

The starting mechanism of the single-phase induction motor is mainly an additional stator winding (start/ auxiliary winding) as shown in Figure 19.2. The start winding can have a series capacitor and/or a centrifugal switch. When the supply voltage is applied, current in the main winding lags the supply voltage due to the main winding impedance. At the same time, current in the start winding leads/lags the supply voltage depending on the starting mechanism impedance. Interaction between magnetic fields generated by the main winding and the starting mechanism generates a resultant magnetic field rotating in one direction. The motor starts rotating in the direction of the resultant magnetic field.

Once the motor reaches about 75% of its rated speed, a centrifugal switch disconnects the start winding. From this point onward, the single-phase motor can maintain sufficient torque to operate on its own. Except for special capacitor start/capacitor run types, all single-phase motors are generally used for applications up to 3/4 hp only. Depending on the various starting techniques, single-phase AC induction motors are further classified as described in the following sections.

Fig. 19.1 Single phase induction motor (without start mechanism)

Fig. 19.2 Single phase induction motor (with start mechanism)

19.2  Types of Single Phase Induction Motor

19.2.1  Split-phase induction motor

The split-phase motor is also known as an induction start/induction run motor. It has two windings: a start and a main winding. The start winding is made with smaller gauge wire and fewer turns, relative to the main winding to create more resistance, thus putting the start winding’s field at a different angle than that of the main winding which causes the motor to start rotating. The main winding, which is of a heavier wire, keeps the motor running the rest of the time.

The starting torque is low, typically 100% to 175% of the rated torque. The motor draws high starting current, approximately 700% to 1,000% of the rated current. The maximum generated torque ranges from 250% to 350% of the rated torque (see Figure 19.8 for torque-speed curve).

Good applications for split-phase motors include small grinders, small fans and blowers and other low starting torque applications with power needs from 1/20 to 1/3 hp. Avoid using this type of motor in any applications requiring high on/off cycle rates or high torque.

Fig. 19.3 Split-phase induction motor

19.2.2  Capacitor start induction motor

This is a modified split-phase motor with a capacitor in series with the start winding to provide a start “boost.” Like the split-phase motor, the capacitor start motor also has a centrifugal switch which disconnects the start winding and the capacitor when the motor reaches about 75% of the rated speed.

Since the capacitor is in series with the start circuit, it creates more starting torque, typically 200% to 400% of the rated torque. And the starting current, usually 450% to 575% of the rated current, is much lower than the split-phase due to the larger wire in the start circuit. Refer to Figure 19.8 for torque-speed curve.

A modified version of the capacitor start motor is the resistance start motor. In this motor type, the starting capacitor is replaced by a resistor. The resistance start motor is used in applications where the starting torque requirement is less than that provided by the capacitor start motor. Apart from the cost, this motor does not offer any major advantage over the capacitor start motor. They are used in a wide range of belt-drive applications like small conveyors, large blowers and pumps, as well as many direct-drive or geared applications.

Fig. 19.4 Capacitor start induction motor

19.2.3  Permanent split capacitor (capacitor run) induction motor

A permanent split capacitor (PSC) motor has a run type capacitor permanently connected in series with the start winding. This makes the start winding an auxiliary winding once the motor reaches the running speed. Since the run capacitor must be designed for continuous use, it cannot provide the starting boost of a starting capacitor. The typical starting torque of the PSC motor is low, from 30% to 150% of the rated torque. PSC motors have low starting current, usually less than 200% of the rated current, making them excellent for applications with high on/off cycle rates. Refer to Figure 19.8 for torque-speed curve.

The PSC motors have several advantages. The motor design can easily be altered for use with speed controllers. They can also be designed for optimum efficiency and High-Power Factor (PF) at the rated load. They’re considered to be the most reliable of the single-phase motors, mainly because no centrifugal starting switch is required.

Permanent split-capacitor motors have a wide variety of applications depending on the design. These include fans, blowers with low starting torque needs and intermittent cycling uses, such as adjusting mechanisms, gate operators and garage door openers.

Fig. 19.5 Permanent split capacitor (capacitor run) induction motor

19.2.4  Capacitor start/capacitor run induction motor

This motor has a start type capacitor in series with the auxiliary winding like the capacitor start motor for high starting torque. Like a PSC motor, it also has a run type capacitor that is in series with the auxiliary winding after the start capacitor is switched out of the circuit. This allows high overload torque.

This type of motor can be designed for lower full-load currents and higher efficiency (see Figure 19.8 for torque-speed curve). This motor is costly due to start and run capacitors and centrifugal switch. Application of these motors include air compressors, high-pressure water pumps, vacuum pumps and other high torque applications requiring 1 to 10 hp.

Fig. 19.6 Capacitor start/capacitor run induction motor

19.2.5  Shaded-pole induction motor

Shaded-pole motors have only one main winding and no start winding. Starting is by means of a design that rings a continuous copper loop around a small portion of each of the motor poles. This “shades” that portion of the pole, causing the magnetic field in the shaded area to lag behind the field in the unshaded area. The reaction of the two fields gets the shaft rotating.

Because the shaded-pole motor lacks a start winding, starting switch or capacitor, it is electrically simple and inexpensive. Also, the speed can be controlled merely by varying voltage, or through a multi-tap winding. Mechanically, the shaded-pole motor construction allows high-volume production. In fact, these are usually considered as “disposable” motors, meaning they are much cheaper to replace than to repair.

 The shaded-pole motor has many positive features but it also has several disadvantages. Its low starting torque is typically 25% to 75% of the rated torque. It is a high slip motor with a running speed 7% to 10% below the synchronous speed. Generally, efficiency of this motor type is very low (below 20%).

The low initial cost suits the shaded-pole motors to low horsepower or light duty applications. Perhaps their largest use is in multi-speed fans for household use. But the low torque, low efficiency and less sturdy mechanical features make shaded-pole motors impractical for most industrial or commercial use, where higher cycle rates or continuous duty are the norm. Figure 19.8 shows the torque-speed curves of various kinds of single-phase AC induction motors.

Fig. 19.7 Shaded-pole induction motor

Fig. 19.8 Torque-speed curves of different types of single-phase induction motors